by Keir Thomas

I’ve just finished writing a piece for Macworld here in the UK called “Create a FrankenMac“. It’s about ways in which older Macs, going back to the first Core 2 Duo models back in 2007, can be boosted so that their day-to-day performance is as good as new models. The feature also looks at how you can get modern Mac features on your old Mac, such as Fusion drive technology, and Handoff/Continuity.

When writing the piece I had to figure out a way to make a DIY Fusion drive on Yosemite. I searched extensively and it appears nobody has written about this. All the instructions I found relate to DIY Fusion drives on Mavericks or earlier versions of OS X, and with the move to default CoreStorage on Yosemite the task is not quite as simple as it used to be. I can’t let the cat out of the bag about how it’s done because that’s what Macworld paid me for but I can tell you of a few experiments I did.

SSD + SSD = Fusion
What if two SSDs are combined together into a DIY Fusion setup under Yosemite? That’s what I did with a Crucial M4 256GB, fitted as the main drive in my MacBook Pro, and a Kingston V300 128GB, fitted where the optical drive used to be. Theoretically there’s nothing to be gained in terms of performance but it seemed to be a simple way of concatenating two drives into one, so that I didn’t have to mess around choosing which disk to store certain files on. Well, I can say that it works and has been reliable so far. It’s at least as fast as when I used the Crucial M4 on its own as a boot drive. It might even be faster but I have trouble accepting that because it doesn’t really make much sense. Most apps start within a second. Bigger apps like Photoshop start within 2-3 seconds. Because I used a Time Machine backup to restore from I also have a recovery partition on the disk, so have been able to enable FileVault too. One concern I had was that OS X might default to the slower Kingston drive, rather than the faster Crucial M4. However, Patrick Stein who initially probed Fusion technology and pioneered the DIY approach says that Fusion “always [chooses] the faster drive“. How Fusion works is entirely undocumented, I believe, but it might be that OS X has some kind of benchmarking function built-in. Whatever the case, I suspect that Fusion might be offered as a function in future versions of Disk Utility as a way of concatenating two or more drives into one volume without RAID. The fact Patrick combined three drives into one using Fusion seems to suggest it has ambitions beyond mere faster rotational hard disks.

RAID stripes
Talking of which, I also tried a RAID stripe setup combining the the two SSDs together. First I tried 32KB blocks (which is default), and then started afresh with 16KB blocks. RAID is setup using Disk Utility from the Recovery Console — you simply drag the two partitions from the disks to the box under the RAID tab heading. The block size can be set by clicking the Options button beneath. Anyway, to cut a long story short, IO performance wasn’t great. Still faster than a traditional rotational disk, but slower than the Crucial M4 had been on its own. Additionally, a recovery partition isn’t allowed on a RAID setup, and this means no FileVault either. Considering that the total size of the RAID array is less than the actual combined size of the two disks – a limitation of the way RAID striping works – then it’s a pretty dumb idea.

Let the hacker beware…
There are a few caveats if you’d like to create a DIY Fusion drive using either two SSDs, or a rotational disk and an SSD. The first is that Patrick Stein reported getting a few system crashes during his testing (kernel panics). However, his testing was done on older versions of OS X and it may be that the bugs have since been fixed. Apple tends to be pretty tight on core technologies. The second warning is that spreading data across more than one drive increases the possibility of drive failure — if either of the two drives fails, then ALL my data is lost. The probability of one of two drives failing is higher than that of a single drive failing, as in a traditional setup. However, I run a tight backup routine so I consider this aspect guarded against, and apps like DriveDx can help monitor underlying drive diagnostics, but it’s worth bearing in mind.

Update July 2015: I had the two SSDs running in Fusion mode, as described above, up until I got a new MacBook in July 2015. That’s about five months. It worked perfectly without a single problem. I really do recommend it as a way of seamlessly and speedily combining two SSDs into one virtual disk.